The present invention relates to a method and apparatus for stretching and shaping the spectrum of an optical pulse. More specifically, the present invention provides a commercially feasible method and apparatus for arbitrarily shaping the spectrum of an optical pulse by first stretching it with a novel long length fiber Bragg grating and then modifying the amplitude of the stretched pulse temporally.
An optical pulse is a brief burst of radiation in the optical wavelength range. Generally speaking, an unchirped pulse of duration t has a spectral width of .about.1/t, e.g. a.about.1 nanosecond (10.sup.-9 second) pulse has a .about.1 GHz (10.sup.9 Hz) spectral width. The time limit on what constitutes a pulse depends on the application for which the pulse is being sampled or utilized. Various methods may be used to produce optical pulses, such as by Q-switching lasers, mode-locking lasers, or rapidly modulating a light signal. As discussed, a pulse of a given duration has a determinable spectral width, and is thus comprised of radiation from a span of wavelengths. Although a pulse is comprised of many wavelengths of radiation, all these wavelengths will travel simultaneously generally through an optical path, possible exceptions are when the pulse is chirped and thus certain wavelengths lag behind others. The method of the present invention separates these different wavelengths in time so that they can be individually accessed and modified.
In several applications, the spectral properties of optical pulses are exploited to perform useful functions. Broad-spectrum optical sources generated by pulsed radiation are important in many applications. In some optical fiber-based communication systems, the cost of individual distributed feedback laser (DFB) sources may be prohibitive, so sources for spectrally sliced and chirped pulse Wavelength-Division Multiplexer (WDM) communication systems, where signals are sent separately over many wavelength channels, are attractive. WDM communication systems are typically added to preexisting systems when increased capacity is needed but the ability for higher speed data transmission is limited. The spectral properties of optical pulses also are critical to applications where the coherence function of the pulse is of interest, such as Optical Coherence Tomography (OCT) and Interferometric Fiber Optic Gyroscopes (IFOGs).
For some of the applications described above and others, the spectrum of a pulse should be broad, as well as smoothly varying and well controlled. In these applications, spectral shape can be as critical as spectral width. In WDM, OCT, and IFOG applications, the shape of the spectrum directly impacts performance. For WDM, channel equalization requires a flat spectrum; for OCT, image resolution is determined by the signal spectrum; and for IFOG, non-Gaussian spectra cause errors due to coherent Rayleigh backscattering.
Generally the initial spectrum of a pulse must be modified to optimize the performance of a given application, such as WDM, OCT, and IFOG.
Time domain spectral shaping (TDSS) is another technique for spectral shaping, where a pulse is chromatically dispersed temporally and then the amplitude of the stretched pulse is modified. The amplitude of the stretched pulse in the time domain is referred to in the present text as the pulse envelope. To achieve reasonable spectral shaping, the pulse envelope is generally stretched by several times its initial duration. With proper calibration and correction factors, the stretched pulse envelope can be correlated to the frequency spectrum of the pulse. Hence, altering the amplitude of the stretched pulse envelope modifies the frequency spectrum of the pulse. The temporal resolution of the optical modulator and electronics used to modify the stretched envelope limits the available wavelength resolution of the spectral shaping. Standard available electronic equipment has a bandwidth that is generally not greater than .about.1 GHz and is thus limited to a .about.1 ns temporal resolution.
Pulse stretching has been demonstrated experimentally by using a long length of optical fiber to stretch the pulse temporally. A 20 km length of standard single mode fiber (SMF), with group velocity dispersion of .about.15 ps/nm, was used to stretch pulses emitted from a mode-locked laser. As the different wavelengths of light that comprised the pulse propagated through the SMF, they traveled at different group velocities and thus were separated temporally at the output of the fiber. The pulse envelope was stretched from &lt;1 picosecond to .about.25 nanoseconds. Because the pulse was chromatically dispersed temporally and the initial pulse envelope was much smaller than the stretched envelope, the stretched pulse envelope was nearly identical to the spectrum shape of the pulse, with proper correction and calibration factors.
In the experiment described above, a photodetector was used to measure the pulse envelope, a reference level was set, and, with an optical amplitude modulator, the spectrum was flattened by forcing all wavelength components to have intensities lower than the reference level. This system was used in a WDM communication system to equalize different channel strengths. This particular implementation of TDSS was limited to flattening spectra and was not used to shape the spectrum arbitrarily. Since this TDSS implementation used a 20 km length of fiber, which is bulky (requires a 4 inch high by 9 inch diameter spool) and displays strong temperature dependent instabilities, this particular technique for stretching pulses is unsuitable for commercial deployment in the previously described applications.
It has been theorized that using a more dispersive element, such as a fiber grating, could increase the amount of pulse stretching and reduce the system size. However, standard chirped Bragg gratings are typically less than 15 centimeters in length, and thus can not provide a meaningful dispersion over a wide bandwidth. Specialized gratings of less than 1.5 m in length have been produced to recompress dispersed light in communication system, but these devices also cannot provide a useful dispersion over a bandwidth wide enough for the above mentioned applications. There are no known instances of an actual application using a fiber Bragg grating to perform time domain spectral shaping of an optical pulse.
Accordingly, the need remains for a commercially viable apparatus and a method for stretching optical pulses and shaping their spectrum.